A novel ultrahigh-vacuum system for gas adsorption has been developed for use with a differential microcalorimeter, in order to generate reliable differential heats of adsorption in the low-coverage region of the heat isotherms. It is in this low-coverage region where the strong Lewis sites of zeolites are thought to be detected. In this work, a well-crystallized H-ZSM-5 zeolite which contains very few Lewis sites has been compared with three other H-ZSM-5 zeolites, which contain increasing amounts of intrinsic strong Lewis sites. True Lewis sites were also induced in all of these zeolites by high-temperature dehydroxylation in vacuum. The differential heats of ammonia adsorption were measured, and the results correlated with data from other techniques such as temperature-programmed desorption of ammonia monitored by mass spectrometry and in situ infrared spectroscopy of adsorbed pyridine. The differential heats of adsorption of ammonia on the activated zeolites yielded information about the number and strength of the strong Lewis and Bronsted acid sites. The use of high-vacuum techniques with microcalorimetry allows for accurate determination of the low equilibrium pressures (down to 0.1 Pa) involved with respect to the adsorption of ammonia. Rather small doses of gas can be admitted, thus allowing the more reproducible measurement of an isothermal heat curve. The use of metallic cells also tends to give a rapid heat conductivity. In H-ZSM-5 zeolites, the strong Lewis sites were found to be associated with differential heats of ammonia adsorption at between 185 and 160 kJ/mol and the Bronsted site acid strength was characterized by heats of the order of 157-140 kJ/mol. An NH4-ZSM-5 zeolite which was activated at 673 K and possessed very few Lewis sites was observed to have very homogeneous Bronsted acid strength distribution, whereas the other zeolites which contained more intrinsic Lewis sites had differing Bronsted acid strengths of a wider energy distribution.